A circumstance which renders the rete mucosum an object of considerable interest, is, that the peculiar com plexion of different individuals is conceived to depend up on the colour of this part ; and this would appear to be the case, whether it be actually a proper membrane, as 'Malpighi supposed, or simply an assemblage of vessels, according to the doctrine of Bichat. It seems as a mat ter of fact, that in the Negro it is black, in the Asiatic yellow or tawny, in the native American reddish brown or copper-coloured, and in the European of different shades of olive. The primary cause of this difference of colour in the rete mucosum has been popularly ascribed to the operation of the sun's rays, as referable to the same ac tion by which the skin is browned in consequence of ex posure to a bright light. But this tanning of the skin has no connexion with the permanent colour of the negro,and probably exists in a different organ. The blackest com plexions are not found in the hottest regions ; and there are considerable tribes, nearly under the equator, whose skin is whiter than many Europeans. Besides, the brown ness of the skin, which is produced by the sun, is not transmitted from parents to their offspring, whereas the children of negroes are equally black, in whatever climate they are born, and their complexion is not altered by a change of climate during any number of successive ge nerations. It has not been precisely ascertained upon what part of the skin the sun acts, but it is probably upon the epidermis, because there are certain appli cations, as blisters and various mild corrosives, which are stated to have the effect of rernoe ing tan, and which, it may be inferred, act principally upon the external sur face.
In connexion with the colour of the skin, we may ad vert to a singular variety of the human species, which is also found among other classes of animals, where the skin is entirely without colour, producing a perfect opaque whiteness. Of this peculiarity, v, hick is supposed to de pend upon the absence of the rete mucosum, we have al ready given an account under the article ALBINO, in an early part of this work ; we shall at present only remark concerning it, that this condition of the skin appears both to confirm the existence of a proper substance, upon which the colour of the complexion depends, and like wise tends to prove that the shade of the complexion is independent of the immediate operation of the sun's rays.
Under the rete mucosum lies the true skin, the cutis or corion, a body of considerable thickness, tough, flexi ble, of a dense texture, composed of a number of small fibres or plates, closely interwoven together. Its exter nal surface is nearly smooth, while internally it is more loose or irregular, as it is connected with the parts below it by the cellular texture, into which substance it passes by almost insensible degrees. Besides the proper mem branous basis of the skin, there is an extensive surface of nerves and blood-vessels, which are connected with it, and which are spread over every part of it with so much mi nuteness, that it is impossible to insert into it even the smallest point of a needle, without both exciting a sen sation and producing a discharge of blood. With the
exception of some of the organs of sense, it perhaps possesses more feeling than any other part of the body, and it is accordingly observed in surgical operations, that the most severe pain is experienced during the division of the skin.
The surface of the skin, when minutely examined, is found to be rendered unequal by a number of little emi nences or projections, which have obtained the name of papilla:. They are said to contain each of them a small branch of an artery and a nerve, of which they constitute the ultimate ramifications, and are supposed .to be the im mediate seat of the organ of touch, and of the other sen sations which reside in the surface of any part of the body. They are observed to be the most numerous, and to be of the largest size, in those parts of the body where the touch is the most delicate, as in the points of the fingers, or in the organs which exercise any peculiar function, as the tip of the tongue.
The minute structure of the skin is described by Haller as consisting of threads or plates, which are short, intri cate, and closely interwoven together, the external parts being more dense, and the internal more spongy. This texture may be easily detected by maceration in water, when a tissue of fibres will be obtained, which are inti mately connected together, with a number of areol2e be tween them, through which the ner' es and vessels pass that ramify on the external surface. Except in this state of partial decomposition it is very difficult, if not impos sible, to exhibit these pores to the eye, yet there can be no reasonable doubt of their existence. The difficulty with which they are rendered. visible, probably, depends upon their not piercing the cutis in a straight direction, but following a winding course, and passing between the folds or plaits, of which the skin is composed.
The properties of the skin may be considered under two points of view, either as depending upon its physical struc ture, or upon its action as a part of the living body, plen tifully furnished with blood-vessels and nerves. The pro perties Of its membranous basis are the same with those of other membranes, and are entirely mechanical, while its blood-vessels and nerves render it subject to many of those changes and actions which are possessed by the parts of the body which have the greatest share of vitality.
Although the chemical composition of the skin has been much attended to by the moderns, our knowledge respect ing it is still imperfect. It is generally described as con sisting-of a solid jelly, differing from jelly as procured from other substances, solely in being more dense and less so luble in water. M. Seguin, who has particularly attended to the nature of the skin, in connection with the process of tanning, describes it as consisting of two distinct struc tures; one which forms its basis, composed of a number of minute interlacing fibres, nearly similar to the muscular fibres, and the other a semi-fluid mucus,or gelatinous mat ter, mechanically interspersed through these fibres. Con sidered generally, the idea of Seguin appears to be sup ported by observation, although it must be admitted that it is somewhat hypothetical.